This invention relates to submarine cable and, more specifically, an optical fiber submarine cable. Further, this invention relates to a method of making an optical fiber submarine cable.
Optical fiber submarine cables have sometimes used a hermetic tube to surround and enclose the optical fibers. The tube prevents moisture from weakening the fiber. The hermetic tube additionally prevents pressure from causing microbending of the fiber which will in turn attenuate or weaken the optical signal. In some submarine cables, the tube is made of highly conductive copper or aluminum and serves to supply electrical power to optical repeaters spaced along the cable. When the tube is made of conductive material, it is enclosed within electrical insulation to insulate it from the water.
Generally, the hermetic tube may be continuously roll formed from flat strip. The tube material is continuously fed and shaped around a continuously fed optical fiber or group of fibers. The seam resulting from the roll forming of the flat strip around the optical fibers is then welded or soldered shut. In either case, the strip must be of relatively low temper or hardness in order to accept the change in shape from flat to tubular. The resulting tube accordingly has a low yield strain such that if the tube is stretched longitudinally, say 0.5%, then the tube will take on a permanent elongation of 0.2% when the tension is relaxed. This causes residual stress to be applied to the optical fibers inside of the tube, thereby greatly decreasing their life expectancy. In addition, if such soft tube is repeatedly subjected to tensile stress, it is prone to cracking or necking down (decrease in diameter due to axial stretching). Either of these will result in reduced electrical conductivity and may cause high local strain on the optical fibers within the hermetic tube.
Typical designs try to avoid yield strain by incorporating a large number of strength members such as steel, Kevlar, or carbon fibers. In order to make the cable so strong that its hermetic tube will not yield under any foreseeable operating conditions, the submarine cables end up being very large and heavy.
The following U.S. patents show various designs which have heretofore been used for cables:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 4,072,398 Larsen et al Feb. 7, 1978 4,097,119 Kumamaru et al June 27, 1978 4,110,001 Olszewski et al Aug. 29, 1978 4,146,302 Jachimowicz Mar. 27, 1979 4,156,104 Mondello May 22, 1979 4,160,872 Lundberg et al July 10, 1979 4,199,224 Oestreich Apr. 22, 1980 4,239,336 Parfree et al Dec. 16, 1980 4,278,835 Jackson July 14, 1981 4,317,000 Ferer Feb. 23, 1982 4,341,440 Trezeguet et al July 27, 1982 4,359,598 Dey et al Nov. 16, 1982 4,371,234 Parfree et al Feb. 1, 1983 ______________________________________
The Larsen et al patent discloses a communication cable using optical fibers and an outer coating. The optical fibers are loosely twisted together and provided with tension-relieving wires and sheathed to form a cable with interspaces filled with lubricants.
The Kumamaru et al patent shows an optical fiber cable using cushioning layers made of nonwoven plastic fabric or foamed plastic. An outer sheathed layer made of a composite of metal and plastic is also used.
The Olszewski et al patent discloses an optical fiber cable construction including a seam welded metalic tube to protect the optical fiber. The optical fiber is placed within a helical channel in a core element.
The Jachimowicz patent discloses an optical fiber cable having optical fibers helically wound within a welded metal tube. When the cable is stretched, the fibers remain relaxed due to their helical construction.
The Mondello patent discloses a submarine cable including optical fibers and cable strength members including a central filament and layers of stranded steel wires separated from the central filament by an insulating core member. A metalic tube surrounds the layers of stranded steel wire and provides a DC path for powering optical repeaters and a hermetic moisture-barrier for the fibers.
The Lundberg et al patent discloses a floating cable system including a metalic protective layer around inner electrical conductors.
The Oestreich patent discloses an optical fiber cable including helical extending chambers in which the optical fibers are disposed. Additionally, an apparatus for constructing the cable is disclosed.
The Parfree et al '336 patent shows an optical fiber communication cable including optical fibers within a copper tube. A plastic layer surrounds the copper tube and strength members are disposed around the plastic layer. An outer sheath surrounds the strength members.
The Jackson patent shows an optical fiber submarine cable wherein the optical fibers are disposed in petroleum jelly within a copper or aluminum tube. Polycarbonate surrounds the tube and in turn is surrouded by an aluminum water barrier. Inner and outer layers of steel wires are wound in opposite directions around the aluminum water barrier. Mylar tape and polyethylene layers surround the outer wire layers. The inner and outer wires press against each other and against the aluminum water barrier to make the cable rigid when it is under hydrostatic pressure.
The Ferer patent discloses a cable including a plurality of electrically conductive wires or a bundle of optical fibers surrounded by an extrusion of plastic. An inner layer of KEVLAR fibers surrounds the plastic and is oppositely wound from an outer helical layer of alternating KEVLAR and nylon fibers separated from the inner layer of wires by a thin plastic film. A braided outer jacket or covering surrounds the outer layer of wires.
The Trezeguet et al patent discloses a submarine optical fiber cable including a drawn copper tube having a longitudinal weld and surrounding the optical fibers. Liquid is disposed within the tube, whereas its outside is surrouded by a polyolefin outer sheath, for example polyethylene.
The Dey et al patent shows an overhead electric transmission system including an optical fiber or optical fiber bundle. Different layers of wires surround a core in which the optical fibers are disposed. Adjacent layers of wires are wound in opposite directions.
The Parfree et al '234 patent discloses a submarine optical fiber cable including optical fiber within an aluminum tube surrounded by an inner layer of armouring wires and a polyetheylene layer. This in turn is surrounded by an additional layer of low density polyethylene having a bedding material such as jute or polypropylene fibers around it. Just outside the jute or polypropylene fiber layer are outer steel armor wires followed by an additional layer of jute or polypropylene fibers.
The following documents disclose various other cable designs:
______________________________________ Document Publication Date ______________________________________ UK Patent Appln. April 21, 1982 2,085,187A UK Patent Appln. March 12, 1980 2,029,048A "Submarine Optical March 1, 1982 Fiber Cable: Development and Laying Results" by Kojima et al - In APPLIED OPTICS, Vol. 21, No. 5, pgs. 815-821. ______________________________________
The UK published patent application No. 2,029,048A discloses an optical fiber submarine cable including a power supplying and pressure resisting layer which may be formed from a tape-like material into a pipe-like configuration. Tension resisting wires are wound around the power supplying pressure resisting layer and in turn are surrounded by insulation. An outer sheath surrounds the insulation. The wire layer is used as a surge attenuator since it is made of lower conductivity material than the power supplying layer.
UK patent application No. 2,085,187A discloses an optical fiber cable construction including one or more optical fibers disposed within a conductive tube and surrounded by reinforcing elements. The reinforcing helically wound elements are in turn surrounded by conductive elements wound in the opposite direction.
The magazine article "Submarine Optical Fiber Cable; Development and Laying Results" discloses various design criteria for submarine optical fiber cable. Additionally, it shows a torqueless armoring structure wherein a cable core is surrounded by inner and outer armoring layers of oppositely wound wires.